In the past, bone defects used to be restored by a technique of transplanting a part of a subject's own bones or a technique of complementing or replacing bones with artificial implants. Such alternative bone implants were required to have properties such as bioadaptability or bone conductivity and bone inductivity (a property of incorporating bone tissue and accelerating bone formation) in addition to mechanical properties similar to those of biological bones. Accordingly, porous materials such as porous ceramics used to be preferably employed as alternative bone implants because they were easily penetrated by bone tissues and they had good bone conductivity and bone inductivity.
Currently commercialized porous hydroxyapatite, however, is nonabsorbable. Even though this hydroxyapatite was porous, cells could not penetrate pores that did not communicate with the exterior because of this nonabsorbability. Accordingly, the final strength was not very high. Also, it was likely to collapse upon moisture absorption during surgical operations. Thus, the use thereof in surgical operation, which required strength, was difficult.
Bone fillers of porous β-TCP, which is bioabsorbable and has porosity of 75%, are commercialized (Osferion®, Olympus Optical Co., Ltd.; average pore diameters: 100 μm to 400 μm). This β-TCP is highly likely to collapse when it is handled. Thus, it was difficult to prepare this material in a form that is adequate for the transplant site, and was difficult to handle since implants easily become detached from the transplant site.
Under the above circumstances, the present inventors have conducted various studies to develop a composite material of hydroxyapatite and collagen that has a structure similar to that of biological bones and to improve its properties. For example, JP Patent Publication (Kokai) No. 7-101708 A (1995) discloses a process for producing a complex of apatite and an organic substance having the Young's modulus similar to that of biological bones with the gradual addition of a mixed solution of collagen and phosphoric acid in a suspension of calcium hydroxide. JP Patent Publication (Kokai) No. 11-199209 A (1999) discloses that a structure similar to that of biological bones can be realized by regulating pH and temperature at the time of reaction. Further, JP Patent Publication (Kokai) No. 2000-5298 A discloses a technique for enhancing the formation of apatite on a collagen surface with the use of organic acid.
In spite of these efforts, however, many complexes of hydroxyapatite and collagen that were obtained by the conventional technique had small pore diameters (approximately 0 μm to 100 μm) and had low porosities (50% or lower).
An object of the present invention is to provide novel porous and composite materials having large pore diameters, high porosities, and mechanical strengths that are suitable for implants such as bone fillers.
The present inventors have conducted concentrated studies in order to attain the above object. As a result, they have found that composite materials having large pore diameters and high porosities could be obtained by gelatinizing a part of the collagen constituting a complex of hydroxyapatite and collagen, followed by freezing and lyophilization thereof. They have also found that application of surface crosslinking to the composite material realized mechanical strengths that were suitable for implants such as bone fillers. This has led to the completion of the present invention.
More specifically, the present invention provides the following (1) to (12).
(1) A process for producing porous and composite materials comprising steps of: cooling a complex containing at least one calcium salt selected from calcium carbonate, calcium phosphate, and hydroxyapatite and collagen, at least a part of which is gelatinized, to gelate the gelatin; freezing it; and then lyophilizing the resultant.
(2) A process for producing porous and composite materials comprising a step of introducing surface crosslinking between collagens to the porous and composite materials obtained by the process according to (1) above.
(3) The process according to (2) above, wherein the step of introducing surface crosslinking is carried out by immersing the porous and composite materials in a solution containing a crosslinking agent.
(4) The process according to any one of (1) to (3) above comprising a step of adding a crosslinking agent to gelatinized collagen to introduce internal crosslinking between collagens.
(5) The process according to any one of (1) to (4) above, wherein the calcium salt is hydroxyapatite.
(6) A process for producing porous and composite materials comprising the following steps of:                1) gelatinizing at least a part of the collagen constituting a complex containing hydroxyapatite and collagen;        2) introducing internal crosslinking between collagens with the addition of a crosslinking agent to the complex;        3) obtaining porous and composite materials by cooling the complex to gelate the gelatin, freezing it, and then lyophilizing the resultant; and        4) introducing surface crosslinking between collagens by immersing the porous and composite materials in a solution containing a crosslinking agent.        
(7) The process according to any one of (2) to (6) above, wherein internal and/or surface crosslinking between collagens is introduced using glutaraldehyde as a crosslinking agent.
(8) The process according to any one of (2) to (7) above, wherein surface crosslinking between collagens is introduced by immersing the porous and composite materials in a solution containing a crosslinking agent comprising ethanol as a solvent.
(9) The process according to any one of (1) to (8) above, wherein the resulting porous and composite materials have porosities of 80% or higher.
(10) The method according to any one of (1) to (9) above using a complex comprising the c-axis of hydroxyapatite being oriented along collagen fibers.
(11) Porous and composite materials having porosities of 80% or higher, which are produced by the method according to any one of (1) to (10) above.
(12) Implants of the porous and composite materials according to (11) above.